However, none of the cases so far seem to be easily spread among people.

“The question again is what would it take for these viruses to evolve into a pandemic strain?” says Marion Koopmans, a virologist at the National Institute for Public Health and the Environment in the Netherlands.

Ever since the H5N1 virus was detected in 1996, scientists have been closely monitoring its development. The bird flu has killed more than 600 people thus far, mostly in Asia.

Professor Koopmans is concerned that scientists were getting no warning until people were reported ill. In the case of H6N1 and H7N9, they didn’t make birds very sick.

“We can surely do better than to have human beings as sentinels,” Koopmans wrote, suggesting that better research and monitoring of birds is needed to prevent the virus from jumping to humans.

Scientists at a pharmaceutical company in Maryland have been testing an H7N9 vaccine. Since it first appeared last spring, the H7N9 virus has infected at least 137 people and killed at least 45.

During the clinical trials, 284 people were given two shots of the vaccine, which appeared to create enough antibodies to protect them.

“They gave a third of the usual dose and yet had antibodies in over 80 percent,” said expert Dr. Greg Poland of the Mayo Clinic. “This is encouraging news. We’ve struggled to make vaccines quickly enough against novel viruses.”

As the different strains of bird flu evolve, researchers are working to stay on top of the virus, but more needs to be done to prevent a global pandemic, doctors say.